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Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending()

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On Wed, Aug 02, 2017 at 01:38:38PM +0200, Peter Zijlstra wrote:
>  	/*
> +	 * The only time this value is relevant is when there are indeed pages
> +	 * to flush. And we'll only flush pages after changing them, which
> +	 * requires the PTL.
> +	 *
> +	 * So the ordering here is:
> +	 *
> +	 *	mm->tlb_flush_pending = true;
> +	 *	spin_lock(&ptl);
> +	 *	...
> +	 *	set_pte_at();
> +	 *	spin_unlock(&ptl);
> +	 *
> +	 *				spin_lock(&ptl)
> +	 *				mm_tlb_flush_pending();
> +	 *				....

Crud, so while I was rebasing Nadav's patches I realized that this does
not in fact work for PPC and split PTL. Because the PPC lwsync relies on
the address dependency to actual produce the ordering.

Also, since Nadav switched to atomic_inc/atomic_dec, I'll send a patch
to add smp_mb__after_atomic(), and

> +	 *				spin_unlock(&ptl);
> +	 *
> +	 *	flush_tlb_range();
> +	 *	mm->tlb_flush_pending = false;
> +	 *
> +	 * So the =true store is constrained by the PTL unlock, and the =false
> +	 * store is constrained by the TLB invalidate.
>  	 */
>  }
>  /* Clearing is done after a TLB flush, which also provides a barrier. */
>  static inline void clear_tlb_flush_pending(struct mm_struct *mm)
>  {
> +	/* see set_tlb_flush_pending */

smp_mb__before_atomic() here. That also avoids the whole reliance on the
tlb_flush nonsense.

It will overstuff on barriers on some platforms though :/

>  	mm->tlb_flush_pending = false;
>  }

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[PATCH -v2 0/4] Getting rid of smp_mb__before_spinlock Peter Zijlstra <peterz@infradead.org> - 2017-08-02 13:50 +0200
  [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Peter Zijlstra <peterz@infradead.org> - 2017-08-02 13:50 +0200
    Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Peter Zijlstra <peterz@infradead.org> - 2017-08-02 15:10 +0200
      Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Mel Gorman <mgorman@suse.de> - 2017-08-02 16:00 +0200
        Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Peter Zijlstra <peterz@infradead.org> - 2017-08-02 16:20 +0200
    Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Mel Gorman <mgorman@suse.de> - 2017-08-02 15:10 +0200
    ARC stuff (was Re: [PATCH -v2 1/4] mm: Rework  {set,clear,mm}_tlb_flush_pending()) Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2017-08-02 15:10 +0200
      Re: ARC stuff (was Re: [PATCH -v2 1/4] mm: Rework  {set,clear,mm}_tlb_flush_pending()) Peter Zijlstra <peterz@infradead.org> - 2017-08-02 15:20 +0200
        Re: ARC stuff (was Re: [PATCH -v2 1/4] mm: Rework  {set,clear,mm}_tlb_flush_pending()) Peter Zijlstra <peterz@infradead.org> - 2017-08-11 16:20 +0200
    Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Will Deacon <will.deacon@arm.com> - 2017-08-03 17:30 +0200
    Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending() Peter Zijlstra <peterz@infradead.org> - 2017-08-11 11:50 +0200

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